A. Field of the Invention
This invention relates to scanning systems which read indicia, for example, barcode symbols, having portions with different light reflectivities and, in particular to optical systems useful in such scanning systems.
B. Description of Related Art
Various optical readers and optical scanning systems have previously been developed for reading barcode symbols appearing on a label or on the surface of an article. The barcode symbol itself is a coded pattern of indicia. Generally, scanning systems electro-optically transform the graphic indicia of the symbols into electrical signals which are decoded into alphanumeric characters. The resulting characters describe the article or some characteristic of the article to which the symbol is attached. Such characters typically comprise input data to a data processing system for applications in point-of-sale processing, inventory control, and the like.
As used in this specification and in the appended claims, the terms "symbol," "barcode," and "barcode symbol" are used to denote a pattern of variable-width bars separated by variable-width spaces. The foregoing terms are intended to be broadly construed to cover many specific forms of one- and two-dimensional patterns, including alphanumeric characters as well as bars and spaces.
The specific arrangement of bars or elements in a symbol defines the character represented according to a set of rules and definitions specified by the code. This is called the "symbology" of the code. The relative size of the bars and spaces is determined by the type of code used, as is the actual size of the bars and spaces. The number of characters per inch represented by the barcode symbol is referred to as the density of the symbol.
To encode a desired sequence of characters, a collection of element arrangements are concatenated to form the complete symbol, with each character being represented by its own corresponding group of elements. In some symbologies, a unique "start" and "stop" character is used to indicate where the barcode symbol begins and ends. A number of different barcode symbologies presently exist. These symbologies include one-dimensional codes such as UPC/EAN, Code 39, Code 128, Codabar, Interleaved 2 of 5, and PDF 417.
Scanning systems have been disclosed, for example, in U.S. Pat. Nos. 4,251,798; 4,369,361; 4,387,297; 4,409,470; 4,760,248; 4,896,026; 4,808,804; and 4,933,538, all of which have been assigned to the assignee of the present invention and which are incorporated herein by reference. As disclosed in some of the above patents, and particularly in U.S. Pat. No. 4,409,470, one existing scanning system comprises a hand-held, portable laser scanning head. The hand-held scanning system is configured to allow a user to manually aim a light beam emanating from the head at a target symbol.
These scanning systems generally include a light source consisting of a gas laser or semiconductor laser. The use of semiconductor devices as the light source in scanning systems is especially desirable because of their small size, low cost and low power requirements. The laser beam is optically manipulated, typically by a focusing optical assembly, to form a beam spot having a certain size at a predetermined optimally located plane comprising the focal plane of the laser beam. Preferably, the cross section of the beam spot at the symbol location approximates the minimum width between symbol regions of different light reflectivity, i.e., the bars and spaces.
In conventional scanning systems, the light beam is repetitively scanned in a line or a series of lines across the symbol by moving a scanning component such as a mirror in the path of the light beam. The scanning component may sweep the beam spot across the symbol, trace a scan line across and beyond the boundaries of the symbol, or scan a predetermined field of view.
The particular location of the symbol depends upon the application of the scanning system. For example, where inventory is to be taken in a warehouse, the operator of the scanning system may be situated a great distance from the object being scanned. In such an application, it is desirable that the optimally located plane be located a great distance from the head of the scanning system. Conversely, in a scanning system used in point-of-sale applications, such as a cash register in a grocery store, it is desirable that the optimally located plane be located immediately outside of the head of the scanning system.
Scanning systems also include a collecting mirror for "collecting" light reflected or scattered from the symbol and directing it to a sensor or photodetector which detects the "collected" light. The photodetector or sensor is positioned in the scanner along an optical path so that it has a field of view which extends at least across and slightly beyond the lateral boundaries of the symbol. A portion of the light beam reflected from the symbol is detected by the photodetector and converted into an electrical signal.
The electrical signal produced by the photodetector is typically converted by a digitizer circuit in the scanner into a pulse-width modulated digital signal having widths corresponding to the physical widths of the symbol elements. The pulse-width modulated digitized signal from the digitizer is decoded, based upon the specific symbology used for the symbol, into a binary data representation of the data encoded in the symbol. The binary data may then be subsequently decoded into the alphanumeric characters represented by the symbol.
A problem arises in the type of systems in which the optimally located plane is located immediately outside of the head of the scanning system. The problem is the occurrence of a so-called "dead zone," which is a region of the scanning beam which is non-decodable.
Thus, in the dead zone, the beam spot to be swept across the symbol is not reliable for scanning. It is desirable to minimize the dead zone and enable even the untrained operator to manipulate the hand-held scanning head properly without reducing the working range or reducing power.
Moreover, the collecting mirror typically used in scanning systems is spherical and has a single focal length. While such collecting mirrors have proven useful, their depth of field is relatively shallow. That is, the "working range," or distance on either side of the optimally located plane at which accurate reading of a symbol can be accomplished, is narrow. Accordingly, it is also desirable to increase the depth of field of presently available laser scanning systems.